1. Field of Invention
This invention relates to elastomeric compression springs. The preferred embodiment of our invention is primarily directed to a compression spring for the bumper assembly of an automobile.
The automotive and insurance industries have long sought a bumper assembly that would absorb the energy of a collision at speeds of 5 miles an hour or less. The acceptability of these units is evaluated by two different criteria. The first is the "no visible damage" criteria. On belief, existing hydraulic bumper assemblies generally meet this criteria, i.e., they protect the vehicle from visible damage. The second is the "no dollar damage" criteria. On belief, the presently used hydraulic units cannot consistently meet the latter criteria. Either the bumper assembly itself or other parts of the automobile suffer functional damage from collisions at 5 mph. Moreover, such bumper assemblies are comprised of a plurality of parts that add undesirable weight, impose assembly costs, require separate, preload springs to minimize vibration, and do not provide a generally constant spring rate.
2. Related Art
Numerous efforts have been made to design a bumper spring for automobiles that would meet the industry goals and criteria. These efforts include U.S. Pat. No. 4,893,857 which issued to General Motors Corporation on an application of Bobinger, et. al. This device of General Motors includes a cylindrical tube containing a first energy absorbing medium, a preload unit comprised of a compression spring and disks and a telescoping piston tube that extends to the bumper. One energy absorbing medium described by General Motors is a thermoplastic material such as Hytrel.RTM., a copolymer elastomer manufactured by E.I. dupont de Nemoirs which has a very high compression set characteristic and which, in part, necessitates the preload assembly to insure that the bumper assembly returns to its original position after a collision.
Another prior effort to design a bumper spring is illustrated by U.S. Pat. No. 4,624,493 which issued to Chrysler Motors Corporation on an application of Hillebrand, et. al. Like the General Motors patent, the Chrysler patent also includes a plurality of parts, i.e., a load bearing tube member with an abutment, a reaction coil spring for preload and an energy absorbing capsule containing the Hytrel.RTM. copolymer elastomer of E. I. dupont de Nemoirs.
Additional prior art related to the present invention is U.S. Pat. No. 4,566,678 which issued to Miner Enterprises, Inc. on an application of David G. Anderson. This patent focuses on the thermoplastic Hytrel.RTM.. It explains a method of producing a hollow spring of Hytrel.RTM. and of eliminating the compression set problems of this thermoplastic. While this patent effectively overcomes the compression set characteristics of Hytrel.RTM. and provides a very effective spring for absorbing substantial energy in rail car applications, modifications are required to apply this concept to the automotive industry in order to meet the "no visible damage" and "no dollar damage" tests. A basic modification is the provision of a different force travel curve. And this, in turn, necessitates design modifications and enhancements that go beyond the teachings of this Anderson patent.